The present invention relates to an ellipsometer at least comprising
light source means which during operation provide a first light beam having a first angular frequency and a second light beam having a second angular frequency,
a neutral beam splitter having a front face for receiving and at least partially transmitting a measurement beam, which, during operation, is produced by scanning a sample with the second light beam, and a rear face for receiving the first light beam, wherein, during operation, the measurement beam interferes with the first light beam at the rear face and an interference beam is thus formed,
a unit for receiving the interference beam, separating the orthogonal components of the interference beam and providing two alternating voltages corresponding thereto.
An ellipsometer of this type is disclosed in International Patent Application WO-A 94/16310 (PCT/NL 93/00283), originating from the Applicant. In said known ellipsometer the light source means comprise a single Zeeman laser which during operation generates two beams shifted somewhat in frequency, which beams are both polarised linearly but perpendicular to one another. With the aid of optical elements known per se, said frequency-shifted beams are split into a first light beam having a first angular frequency and a second light beam having a second angular frequency. The second light beam is used to scan a sample, as a result of which a measurement beam is produced which is incident on the front face of the non-polarising beam splitter. The first light beam is incident on the rear face of the non-polarising beam splitter. The measurement beam is at least partially transmitted through the non-polarising beam splitter, such that interference with the first measurement beam occurs at the rear face of the non-polarising beam splitter. The interference beam is fed to suitable means for analysing the interference beam in order thus to provide desired ellipsometric parameters.
The use of Zeeman lasers is relatively expensive. Moreover, the difference between the frequencies of the light beams generated by the Zeeman laser is determined by parameters set at the time of production. The difference between these frequencies can no longer be adjusted during operation. This constitutes a barrier to the use of the known ellipsometer for some applications.
U.S. Pat. No. 5,396,361 describes a method for stabilising the difference between two optical frequencies, which method can be used for optical heterodyne or homodyne communication. Various systems for carrying out said method are also described. The known system in general comprises two laser sources for generating a first and a second laser beam. The first and second light beams are combined and a portion of the combined beam is subjected to optical heterodyne detection with the aid of an optical detector and then converted into an electrical signal. Said electrical signal is fed to a first input of a multiplier. A second input thereof is connected to an oscillator which provides a reference signal. By means of multiplication, a signal is produced which has a difference frequency component which is a measure of the difference between the frequencies of the electrical signal and the reference signal. The output of the multiplier is coupled to a frequency discrimination circuit, which detects a change in the difference frequency. The output of said frequency discrimination circuit is coupled to at least one of the two laser sources in order to make the difference between the optical frequencies of the lasers substantially constant.
The applications described and suggested by said U.S. Pat. No. 5,396,361 are all in the field of optical communication systems. Applications in the field of ellipsometry are neither described nor suggested.
An objective of the present invention is to provide an ellipsometer with which samples can be scanned in a simple and relatively inexpensive manner.
In order to achieve this objective, the invention provides an ellipsometer as described in the preamble, which ellipsometer is characterised in that the light source means comprise two separate laser sources and in that the ellipsometer is also provided with a control circuit having at least one input for receiving information with regard to either the first and second angular frequencies or the difference between the first and second angular frequencies and at least one output, coupled at least to one of the two laser sources, for controlling the difference between the first and second angular frequencies.
In principle, an inexpensive optical read system can be constructed by using such an ellipsometer instead of a large and expensive Zeeman laser, as in the prior art. xe2x80x9cFrequency mixingxe2x80x9d, which occurred when a Zeeman laser was used, is prevented by the use of two different lasers. Consequently: an ellipsometer having a greater absolute accuracy is obtained.
The two different lasers are preferably frequency-stabilised lasers. The difference frequency is obtained by heterodyne mixing in the optical set-up, which difference frequency can be kept at a substantially constant value with the aid of an electronic control circuit which, for example, is based on the principle of the phase-locked loop (PLL).
Compared with the set-up in which a Zeeman laser is used, the number of components can be reduced. For example, polarising beam splitters are no longer needed. Furthermore, the phase difference between the two electrical signals can be determined inexpensively in any known way. This can be effected, for example, using a bridge circuit in which four diodes are incorporated. A phase detector of this type is relatively inexpensive and not sensitive to the precise frequency of the two electrical signals. It is therefore not necessary to impose stringent requirements on the control circuit, with the result that the price thereof can remain relatively low.
The light sources can be diode lasers, for which the frequency of the transmitted laser beam can be controlled. Diode lasers of this type are now in development. The size of the ellipsometer according to the invention can be limited using diode lasers of this type.
In principle, a set-up can be chosen in which the second light beam first scans the surface of the sample before it is incident on the front face of the neutral beam splitter. In a preferred embodiment, however, the ellipsometer according to the invention is characterised in that the front face of the neutral beam splitter is set up to receive the second light beam, at least partially reflect the second light beam in the direction of the sample to be scanned and then receive and at least partially transmit the measurement beam originating from the sample to be scanned.
In one of the embodiments according to the invention, one of the two alternating voltages is fed to the input of the control circuit. Specifically, both alternating voltage signals have a component having an angular frequency which is equal to the difference between the angular frequencies of the first light beam and the second light beam. The difference between these two angular frequencies is thus fed via an electrical signal to the input of the control circuit.
In an alternative set-up, an electrical signal is not fed to the input of the control circuit but the difference between the frequencies of the first light beam and the second light beam is determined by the control circuit itself in that said control circuit receives at its input a first input signal derived from the first light beam and a second input signal derived from the second light beam. A control circuit of this type can be in the form disclosed in the cited U.S. Pat. No. 5,396,361, the optical signals themselves being fed to the control circuit. In the present invention, portions of the first and second light beams can be deflected with the aid of beam splitting techniques known per se and fed to such a control circuit.
The ellipsometer according to the invention has an interesting application in scanning optical information carriers, such as optical discs. For an application of this type, the ellipsometer according to the invention is characterised in that it is provided with a support for supporting such an optical information carrier.
With the known techniques for reading optical information from an optical disc use is made of amplitude differences in measurement beams originating from the optical disc, which measurement beams are produced by exposing the optical disc to a laser beam. With the ellipsometer according to the invention the optical information from an optical disc can be determined with the aid of phase difference techniques, which can provide more accurate information than can amplitude difference techniques.
The essential difference between the known read techniques and the technique of reading with the aid of an ellipsometer is that in the prior art differences in the polarisation state of reflected or transmitted light as a consequence of written bits of information on the optical disc are converted into amplitude changes in a detected signal, which amplitude differences then have to be measured. In the case of the technique proposed here, the information is present as modulation, specifically as phase difference, on a carrier wave which in fact consists of two carrier waves. In principle, a low-noise system can be made in this way because, just as in a radio system, it is possible selectively to tune to the carrier wave frequency, that is to say the difference frequency between the first and second light beams. The detected alternating voltages can, for example, first be filtered by means of a narrow band bandpass filter which transmits only the difference frequency including a small side band, in which the modulation is contained.